Optical storage system using an antenna for recording information data to a phase-change type medium

ABSTRACT

An assembly and method for recording and/or reading high-density data includes a phase change media, an antenna placed adjacent the phase change media, and a source of electromagnetic radiation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel method and assembly forrecording data at a very high density on phase-change media andphase-change media structures for this novel method.

2. Description of the Related Art

Conventional methods for reading and writing on a media include opticalmethods such as CD-ROM and magneto-optical (MO) storage devices. Byfocusing a laser beam to about one micrometer onto the storage media,one can read and sometimes record/erase information.

The density of an optical memory device is mainly diffraction limited.Specifically, with a focusing lens having a high numerical aperture, thecorresponding radius of the focused beam spot cannot be smaller thanhalf the illumination wavelength. Current commercial devices are alreadyclose to this resolution limit.

Magneto-optical recording methods use a focused laser beam as well tocreate a hot spot on a magnetic medium. The magnetic media, in turn,typically comprises a thin film magnetic media, which, at ambienttemperature, has a high magnetic coercivity and is non-responsive to anexternally applied magnetic field.

A conventional method for reading high density bits and apparatustherefor includes decoding high density data encoded in a digitalrecording media as a series of tags comprising an information bitpattern including a tracking bit pattern.

Several approaches have been demonstrated to improve on the resolution.First, more efficient lasers emitting shortened wavelengths may be used.Typically, a reduction of wavelength by a factor of two can be expectedto provide a four fold increase in data storage density. However, thelasers required for such high storage densities are very expensive andfurther advances by reduction in the wavelength are not in theforeseeable future.

An approach for overcoming the diffraction limit uses evanescent waves,which can be confined due to their non-propagating properties todimensions significant less than the wavelength of the laser. However,these methods often have poor signal-to-noise ratio, reliability andspeed.

SUMMARY OF THE INVENTION

In view of the aforementioned and other problems, drawbacks, anddisadvantages of the conventional methods and structures, an object ofthe present invention is to provide an assembly, a method and a mediastructure in which superior recording on a phase change media isrealized.

An object of this invention is to provide a method and assembly forrecording/reading, on phase change media, bits with a size substantiallysmaller than a conventional focused laser spot.

Another object is to provide a media structure which is a phase-changemedia, which provides superior storage densities.

Co-pending, co-authored U.S. patent application Ser. No. 09/540,726filed on Mar. 31, 2000, entitled ASSEMBLY AND METHOD SUITABLE FORTHERMO-MAGNETIC WRITING/READING OF DATA incorporated herein in itsentirety, discloses an assembly for writing/erasing on a thermo-magneticrecording media as a series of tags including a magnetic information bitpattern. The assembly includes: 1) an antenna placed near athermo-magnetic media; 2) a source of electromagnetic radiation at leasta portion of which can be coupled to the antenna; and 3) means forcoordinating a mutual positioning of the source of electromagneticradiation and the antenna so that the antenna can generate a highlylocalized electromagnetic field in the vicinity of the media forinducing localized heating of the media.

In contrast to the apparatus disclosed in the above-mentioned co-pendingpatent application, in the present invention, the media is notnecessarily magnetic, but rather a phase change media. The presentinvention provides a better solution through the use a phase changemedia instead of a magnetic media.

In an exemplary embodiment of the invention, the phase change media hasa thermal conductivity which is typically 10× smaller than a magneticlayer which greatly improves the recording density.

In a first aspect of the present invention, an assembly for recording(writing/erasing) high-density data, includes a phase change mediahaving a thermal conductivity which is lower than or equal to thesubstrate; an antenna placed near the phase change media; and a sourceof electromagnetic radiation.

In a second aspect of the present invention, a method of recording(writing/erasing) high-density data, includes providing anelectromagnetic wave; providing a phase change media having a thermalconductivity which is lower than or equal to the substrate; andpositioning an antenna near the phase change media for coupling theelectromagnetic wave with the phase change media.

In a third aspect of the present invention, an assembly forwriting/erasing/reading high-density data on a recording media as aseries of tags. The assembly includes a phase change media, an antennapositionable near the media, a source of electromagnetic radiation forproducing an incident wave at least a portion of which may be coupled tothe antenna, and a positioning device that coordinates mutualpositioning of the source of electromagnetic radiation and the antennaso that the antenna can generate a highly localized electromagneticfield in the media.

In an exemplary embodiment of the invention, the antenna is capable ofrecording (writing/erasing) by generating a strong highly localizedelectromagnetic field in the media for inducing local heating of themedia and which is capable of reading by probing the phase with a highlylocalized electromagnetic field.

In another exemplary embodiment of the present invention, the antennagenerates a highly localized field in the media for reading theinformation pattern at low power of the source of eletromagneticradiation and/or b) recording information patterns by inducing localheating at high power of the source of electromagnetic radiation.

Another advantage of this novel method is that it is compatible with amethod for reading high density bits as disclosed in U.S. Pat. No.5,602,820.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of preferredembodiments of the invention with reference to the drawings, in which:

FIG. 1 shows an exemplary embodiment of an apparatus 100 which iscapable of performing a method in accordance with the present invention;

FIG. 2 shows a cross-section of power dissipation in a Co/Pt magneticfilm;

FIGS. 3A and 3B show the effects provided by the incident drive beamfrom FIG. 2 on a conventional thermo-magnetic media and a phase changemedia in accordance with an exemplary embodiment of the presentinvention;

FIG. 4 shows a second exemplary apparatus 400 for performing a method inaccordance with the invention; and

FIGS. 5A and 5B show one exemplary embodiment of a slider which may forma part of an apparatus of FIG. 1 and/or FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawings, there are shown exemplary embodiments ofthe assembly, the methods and the structures according to the presentinvention.

In overview, the present invention can circumvent the diffraction limitby near-field methods. Preferably, an antenna including a nanometricscattering solid (or scatterer) may be excited with an electromagneticfield such as a laser beam, to thereby generate an enhanced and highlylocalized electromagnetic field in the near vicinity of a recordingmedia. By optimizing shape, size, and material type of the antenna andthe phase-change recording media, as well as choosing the best geometry,polarization, and wavelength for the exciting electromagnetic field, onecan generate an electromagnetic near-field source with a substantiallylarger magnitude than the original exciting field. This near-field isalso highly localized to the immediate area near the antenna. Thestrongly enhanced and highly localized electromagnetic near-field caneither heat the phase-change recording media, in order to writeinformation patterns, or, at lower power settings, it can be used forreading by probing the local optical properties of the recording media.

From a physics point of view, this enhancement is due to the oscillatingcharges within the antenna. At some point during the oscillation,charges accumulate at the end of the antenna and produce an enhancednear-field. In a slightly different way of looking at this, the incidentfield lines should match the boundary conditions of the antenna andphase change recording media. At a sharp point all the field lines areconcentrated and give rise to an enhanced near-field at the end of theantenna, which acts as a lightening rod for the incident wave. Thiseffect is enhanced when the gap between antenna and recording media iskept small, which is generally the case for the gap between arecording/reading head and the phase change media in phase-changerecording.

In addition to this geometric enhancement, electromagnetic resonance inthe antenna/phase change media system can further enhance near-fields.Antenna-like resonance, obtained by optimizing the geometry of thematerials (especially the length of the antenna), or surface plasmonresonance (charge density waves), excited by operating at certainfrequency ranges for selected types of material (for example, silver,gold and aluminum) with given shape and size, can further induceenhancement.

Theoretical calculations have shown that this enhanced near-fieldextends substantially into the media, even if it is conducting. Heat istherefore generated throughout the phase change media, in a directmanner. Because of the small dimensions involved, time constraints forheating and cooling are extremely fast (nanosecond) and operatingfrequencies may reach into the Ghz regime

Furthermore, by appropriately choosing the antenna and the phase-changemedia material, one can maximize heat deposition in the film andminimize heating the antenna (and therefore the recording head).

FIG. 1 shows an exemplary embodiment of an apparatus 100 which may beadapted to perform a method in accordance with the present invention.The apparatus 100 includes an antenna 102 (e.g., a nanometric antenna),an electromagnetic source 104, a phase change recording media 106, and aposition controller 108. The antenna 102 is adapted to amplify theincident electromagnetic field 120, preferably in a near-field zone 122.The geometry and material type for this antenna 112 may be modified toensure maximum field enhancement as disclosed in co-pending, co-authoredU.S. patent application Ser. No. 09/540,726 filed on Mar. 31, 2000.

The antenna 102 is excited by an incident electromagnetic field 120,such as can be provided by a laser beam having proper direction, focus,polarization and wavelength as is described in co-pending, co-authoredU.S. patent application Ser. No. 09/540,726 filed on Mar. 31, 2000. Theassociated illumination optics (not shown) can use conventional bulkoptical components (e.g. lenses, mirrors) or integrated components (e.g.optical fibers, optical micro-strips). The choice of the wavelength mayrequire matching to the length of the antenna. In a typical application,the source or laser may be modulated in order to write the informationon the phase-change recording media.

The controller 108 is adapted to mutually position the antenna 102 tothe phase change media 106. One exemplary embodiment of the apparatus100, incorporates the antenna 102 with a sensor (not shown) directly inthe head of a conventional recording system as we know it from magneticrecording.

In one exemplary embodiment, an electronic system and/or air bearingmechanism can control the position and height of the head over the phasechange recording media. The small gap between the head and the phasechange recording media contributes advantageously to the need ofpositioning the antenna at a close distance from the phase changerecording media (typically 20 nm or less).

An exemplary embodiment of this invention is based on aperture-lessnear-field enhancement, used for the purpose of locally heating a phasechange media. Detailed finite element calculations (e.g., Y. Martin etal. J. Appl. Phys. 89, 5774 (2001)) as well as experimental data (Y.Martin et al. J. Appl. Phys. 91, 3363 (2002)) have demonstrated thathighly localized and strong near-fields in the vicinity of correctlydesigned and properly driven antennas can be generated. However, the“stray” heating from the drive laser remains a major problem inconventional systems.

In order to illustrate this problem, the inventors calculated thedissipated power in a Co/Pt magnetic film, which interacts with thenear-field of the antenna 102 (see FIG. 1). In this example, theincident drive beam is radial in order to provide polarization along theelongated probe axis and has a diameter of 0.8 micrometer. The graph ofFIG. 2 shows a cross-section of the dissipated power, which reveals twodistinct parts. The sharp peak in the middle (˜30 nm) represents theenhanced near-field of the antenna, while the long tails at both sidesarise from the drive beam. Thus, the “stray” heating from the drivelaser remains a major problem in conventional systems.

FIGS. 3A and 3B show the effects provided by the incident drive beamfrom FIG. 2 on a conventional thermo-magnetic media and a phase changemedia in accordance with the present invention, respectively. FIG. 3Ashows the steady state temperature distribution for a typicalperpendicular magnetic recording media (10 W/mK, 10 nm thick) on a glasssubstrate (1.2 W/mK). This temperature distribution illustrates that theheating of the localized near-field can be severely smeared out by theheating due to the drive beam. This smearing results in a very poorcontrast ratio between the near-field region and the surrounding regionof the media.

Additionally, the inventors calculations have demonstrated that anincident laser power of 3.5 mW may be required to raise the peaktemperature in the Co/Pt sample by 200 K.

FIG. 3A shows a simple physical picture for the degraded contrast ratiobetween near-field and the surrounding media regions. The temperatureprofiles in FIGS. 3A and 3B result from the heating with two heat spotsources: a small source with diameter d_(NF)˜30 nm (see FIG. 2), causedby the near-field intensity (I_(NF)) under the antenna, and a largesource with diameter d_(FF)˜800 nm caused by the intensity (I_(FF)) fromthe far-field of the focused laser beam. The ratio of the intensity ofthese two sources, obtained from electromagnetic finite-elementcalculation, is the ratio from the peak height to the side-lobe heightin FIG. 2.

For simplification of the calculations for this example, the inventorsalso assumed that the near-field heat source mostly interacts with thethin film (which has a thermal conductivity 1_(film)) while thefar-field heat spot mostly heats the substrate (which has a thermalconductivity 1_(substrate)) With these simplifying assumptions, thetemperature ratio in the near-field (T_(NF)) versus the far-field(T_(FF)) can be approximated by the formula:T _(NF) /T _(FF)=(d _(NF) /d _(FF))(I _(NF) /I _(FF))(I _(substrate) /I_(film))   (1)

Three ratios govern the magnitude of T_(NF)/T_(FF). The first ratio,d_(NF)/d_(FF), is determined by fixed geometry: d_(NF) corresponds tothe desired bit size, and d_(FF) corresponds to the smallest opticalfocused spot that one can produce (about half the optical wavelength).The second ratio, I_(NF)/I_(FF), is determined by the physics ofnear-field electromagnetism; under an optimum and practicalconfiguration, the inventors found a value of about 20 for this ratio.The third ratio, 1_(substrate)/1_(film), is a ratio in thermalconductivity.

With appropriate choice of materials, the inventors have discovered thatone can substantially modify the thermal conductivity ratio to maximizethe peak temperature and T_(NF)/T_(FF) in order to produce a highlylocalized heat spot on the phase change media in comparison withconventional magnetic media. The difference between a magnetic media anda phase change media is explained below.

Thermal profiles in conventional magnetic thin film media are shown inFIG. 3A, for three different substrates. The ratio1_(substrate)/1_(film) increases from the glass substrate 302(1_(substrate)/1_(film)˜0.1), to the quartz substrate 304(1_(substrate)/1_(film)˜1), and to the silicon substrate 306(1_(substrate)/1_(film)˜10), and is accompanied by an increase in thetemperature contrast, T_(NF)/T_(FF). However, this increase comes at aprice, namely the required incident laser power.

For a 200 K peak temperature, the required laser power increases fromapproximately 3.5 mW for glass 302, to 20 mW for quartz 304 and to 90 mWfor silicon 306. Such a large power is impractical and is presently tooexpensive. A typical laser power in an optical drive is in the order ofa few milliwatts. The requirement to obtain a high contrast ratioT_(NF)/T_(FF) with a low laser power limits the applicability of thethermo-magnetic media disclosed in the above-mentioned U.S. patentapplication Ser. No. 09/540,726 filed on Mar. 31, 2000. The presentinvention provides a better solution through the use a phase changemedia instead of a magnetic media.

In the case of a phase-change media (e.g., formed of one or more ofchalcogenides, GaSb, InSb, GaSeTe, AgInSbTe, Sb₂Te₂Ge₅, etc), the phaseof the media may be changed by heating. As an example, a fast and strongheat pulse may convert the media from a crystalline phase to anamorphous phase. In this process, the phase change media is melted andthen kinetically trapped in a thermodynamically less stable amorphousphase (˜500° C.). The media can then be converted back to thecrystalline phase by applying a slightly longer (˜100 ns) but weakerheat pulse (˜200° C.).

An advantage of phase-change media is its very low thermal conductivity.In one exemplary embodiment the thermal conductivity for a phase-changemedia may be approximately 0.6 W/mK in the crystalline phase (more thana factor 10 less than a very thin magnetic film) and approximately 0.2W/mK in the amorphous phase. As a result, the contrast ratio betweenfar-field and near-field heating can be increased without a high laserpower cost.

Thus, as shown in FIG. 3B, the temperature profiles 308, 310 and 312 fora 10 nm thick phase-change media for three different substratesdemonstrates a significantly improved contrast ratio as well as moderatepower requirements.

FIG. 4 shows a second exemplary apparatus 400 for performing a method inaccordance with the invention. The second exemplary apparatus 400includes some additional components and capabilities over the firstexemplary apparatus 100.

In the apparatus 400 the antenna for recording (writing/erasing) and forreading is preferably a single element 406. For recording, the field ofan incident laser is coupled to the antenna 402. Antenna 402 generates astrong local near-field which interacts with the phase-change media 404and provides local heating of the media 404. Modulating the power ofthis laser in accordance with a data signal results in the recording(writing/erasing) of the data signal in the media 404.

For reading, the incident laser power is reduced so that the near-fieldand its interaction with the phase-change media does not alter the phaseof the storage media. Rather, the sensor 406 detects the laser lightscattered by the near-field of the optical antenna. The sensor 406 mayuse interferometric methods to detect the data signal embedded in themedia. These interferometric methods measure the scattered field fromthe near-field interaction between the antenna and the phase-changemedia. The sensor 406 is sensitive to the differences in the index ofrefraction between amorphous and crystalline regions in the phase-changemedia. Thus, the data signal may be determined based upon the sensing ofthese regions.

FIG. 4 also shows a position controller 408, which enables the assemblyto read data written on the phase change media 404 by coordinating theinitial positioning of the magnetic sensor 406 and the phase changemedia 404. Read out methods may include any sensor, such as a near-fieldoptical sensor or electrical sensor. The near-field sensor may be thesame element as the optical antenna used for recording; for example, thesensor and the antenna may be an atomic force microscope tip.

FIGS. 5A and 5B show one exemplary embodiment of a slider 500, havingsimilar gliding properties as a slider for a conventional magneticrecording head. The slider 500 includes an antenna (scatterer) 502, anda lens 504. The slider 500 (or part of it) may be made out oftransparent material, like glass, or alumina, or plastic. The scatterer502 may be a small strip of metal inside the transparent slider 500,positioned close to an air-bearing surface 506 of the slider 500.

The scatterer 502 may be lithographically defined, using techniquessimilar to those for the fabrication of conventional thermo-magneticrecording heads.

Similarly, the lens 504 may be a lithographically defined integratedlens. The focal point of the lens 504 is defined to be at-theintersection of the air bearing surface 506 and the scatterer 502. Alaser beam 508 is directed toward the lens 504 and is focused at the airbearing surface 506 on the end of the scatterer 502. For this purpose, aminiature laser diode (not shown) can be directly mounted on (or above)the slider 500.

Alternatively, the laser light 508 may be provided via an optical fiber(not shown). A polarization controlling element 510 may convert thelaser beam polarization into a radial polarization.

While the invention has been described in terms of several preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Further, it is noted that Applicant's intent is to encompass equivalentsof all claim elements, even if amended later during prosecution.

1. An assembly for at least one of recording and reading data,comprising: a phase change media; an antenna placed adjacent said phasechange media; and a source of electromagnetic radiation to provide anincident wave of electromagnetic radiation to drive said antenna so asto produce non-propagating, near-field electromagnetic radiation fromsaid antenna, wherein said antenna comprises an elongated antenna havinga longitudinal axis that is substantially parallel to a directionextending from said antenna toward said phase change media, wherein saidphase change media has a thermal conductivity which is lower than orequal to a substrate supporting said phase change media, wherein saidphase change media has a thermal conductivity of approximately 0.6 W/mKin a crystalline phase.
 2. An assembly for at least one of recording andreading data, comprising: a phase change media; an antenna placedadjacent said phase change media; and a source of electromagneticradiation to provide an incident wave of electromagnetic radiation todrive said antenna so as to produce non-propagating, near-fieldelectromagnetic radiation from said antenna, wherein said antennacomprises an enlongated antenna having a longitudinal axis that issubstantially parallel to a direction extending from said antenna towardsaid phase change media, wherein said phase change media has a thermalconductivity which is lower than or equal to a substrate supporting saidphase change media, wherein said phase change media has a thermalconductivity of approximately 0.2 W/mK in an amorphous phase.
 3. Amethod of at least one of recording and reading data, comprising:providing a source of electromagnetic radiation; providing a phasechange media; and positioning an antenna adjacent said phase changemedia which receives said electromagnetic radiation so as to producenon-propagating, near-field electromagnetic radiation, wherein saidantenna comprises an elongated antenna having a longitudinal axis thatis substantially parallel to a direction extending from said antennatoward said phase change media, wherein said phase change media has athermal conductivity which is less than or equal to a substrate, whereinsaid phase change media has a thermal conductivity of approximately 0.6W/mK in a crystalline phase.
 4. A method of at least one of recordingand reading data, comprising: providing a source of electromagneticradiation; providing a phase change media; and positioning an antennaadjacent said phase change media which receives said electromagneticradiation so as to produce non-propagating, near-field electromagneticradiation, wherein said antenna comprises an elongated antenna having alongitudinal axis that is substantially parallel to a directionextending from said antenna toward said phase change media, wherein saidphase change media has a thermal conductivity which is less than orequal to a substrate, wherein said phase change media has a thermalconductivity of approximately 0.2 W/mK in an amorphous phase.